Abstract

Piezopolymer-based hydrophone arrays consisting of 20 elements were fabricated and tested for use in measuring the acoustic field from a shock-wave lithotripter. The arrays were fabricated from piezopolymer films and were mounted in a housing to allow submersion into water. The motivation was to use the array to determine how the shot-to-shot variability of the spark discharge in an electrohydraulic lithotripter affects the resulting focused acoustic field. It was found that the dominant effect of shot-to-shot variability was to laterally shift the location of the focus by up to 5 mm from the nominal acoustic axis of the lithotripter. The effect was more pronounced when the spark discharge was initiated with higher voltages. The lateral beamwidth of individual, instantaneous shock waves were observed to range from 1.5 mm to 24 mm. Due to the spatial variation of the acoustic field, the average of instantaneous beamwidths were observed to be 1 to 2 mm narrower than beamwidths determined from traditional single-point measurements that average the pressure measured at each location before computing beamwidth.

The extraordinary technical and medical importance of this paper is based on the type of measuring the shock wave field of an electro-hydraulic (EL) shock wave generator.

For the first time authors try to focus on the variability, the stochastic nature of EL-shock wave generation and its impact on characteristic parameters such as peak pressure and width of the focal zone. Whereas electro-magnetic and piezo-electric shock wave generation usually feature a good shock to shock reproducibility, EL-spark generation varies significantly. Peak pressure recordings may be in a range of 10-60 MPa (see Fig 5a of the paper) while the averaged value, which is usually taken for comparison, is around 30 MPa. These data determine further important calculations of the focal width and energy values required for technical characterization.

The paper describes a linear array hydrophone with 20 elements for single-shot measurements of acoustic fields. It allows single-shot measurements of focal width instead of averaging multiple sequential shots at different spatial locations. Not only large variations in peak pressure but also lateral shifts of up to 5 mm away from the axis could be recorded. These measurements were performed with new (refurbished) electrodes at 14, 17 and 20 kV showing an increased variation with higher voltages applied. I assume an even further jitter with increasing wear of the electrodes as usually seen in clinical use.

The width of single-shot measured beam is consistently 1-2 mm narrower than it would be measured by a single-element hydrophone scanning the field over multiple shock waves.

The results of this study will have an impact on the on-going debate of focal size regarding clinical efficiency.

The extraordinary technical and medical importance of this paper is based on the type of measuring the shock wave field of an electro-hydraulic (EL) shock wave generator.
For the first time authors try to focus on the variability, the stochastic nature of EL-shock wave generation and its impact on characteristic parameters such as peak pressure and width of the focal zone. Whereas electro-magnetic and piezo-electric shock wave generation usually feature a good shock to shock reproducibility, EL-spark generation varies significantly. Peak pressure recordings may be in a range of 10-60 MPa (see Fig 5a of the paper) while the averaged value, which is usually taken for comparison, is around 30 MPa. These data determine further important calculations of the focal width and energy values required for technical characterization.
The paper describes a linear array hydrophone with 20 elements for single-shot measurements of acoustic fields. It allows single-shot measurements of focal width instead of averaging multiple sequential shots at different spatial locations. Not only large variations in peak pressure but also lateral shifts of up to 5 mm away from the axis could be recorded. These measurements were performed with new (refurbished) electrodes at 14, 17 and 20 kV showing an increased variation with higher voltages applied. I assume an even further jitter with increasing wear of the electrodes as usually seen in clinical use.
The width of single-shot measured beam is consistently 1-2 mm narrower than it would be measured by a single-element hydrophone scanning the field over multiple shock waves.
The results of this study will have an impact on the on-going debate of focal size regarding clinical efficiency.
Othmar Wess